Data storage devices are known as devices using different types of recording media such as optical disks, optical magnetic disks or flexible magnetic disks, etc. Among these storage devices, computer storage devices called hard disk drives (HDD) are widely used and are indispensable as storage devices in current computer systems. Moreover, hard disk drives are not limited to computer systems and applications including removable memories allow use in video recorders and players, car navigation systems, cellular telephones or digital cameras. The excellent features offered by HDD will ensure an expanded range of applications.
The magnetic disk used in the HDD contains multiple tracks formed concentrically. Data regions and servo regions are formed on each track. The servo regions contain the servo data (servo patterns) and are formed separately along the track circumference. Data regions contain user data and are formed between one servo region and next servo region. A thin-film device functioning as the magnetic head, writes the user data or reads the user data by accessing the desired data region (address) according to the servo data.
Each servo pattern (called, product servo patterns in these specifications) is made up of a cylinder ID, sector numbers, and burst patterns, etc. The cylinder ID indicates the address of the track, and the sector number indicates the sector address within the track. The burst pattern contains magnetic head position information relative to the track.
The product servo patterns are formed in multiple sectors formed circumferentially and at a separate distance on each track as described above. The product servo patterns are arrayed along the circumference at the same position, or in other words, product servo patterns with the same sector number are arrayed at positions (phases) along the circumference across the entire track. The product servo pattern is written on the magnetic disk at the factory prior to HDD product shipment. An external device called a servo track writer is typically utilized to write the servo track data. In this process, the HDD is set in the servo track writer, a positioner (external positioner mechanism) then positions the head on the HDD in the servo track writer, and a product servo pattern generated by the product servo pattern generator circuit is written on the magnetic disk.
The process for writing the product servo pattern (hereafter, servo write process) occupies a lead position in terms of cost in the HDD manufacturing process. Competition to produce HDD with ever larger storage capacity has become particularly fierce in recent years, resulting in an ever larger number of tracks per inch (TPI). A larger TPI results in a greater number of tracks, so the track width (track pitch) becomes smaller. This smaller track width drives up the servo write process cost because more time is needed for servo writing and the servo writer must have greater precision due to the greater track density. Efforts are being made to reduce this cost and include methods for lowering the servo writer cost and reducing the servo writing time.
One method proposed to resolve the above problems is called Self Servo Write (or SSW). Unlike previous servo write methods, SSW utilizes only the basic mechanism within the HDD unit. In this SSW method, an external circuit regulates the spindle motor (SPM) and the voice coil motor (VCM) within the HDD, and writes the product servo pattern. The SSW method in this way attempts to reduce the servo track writer cost.
The SSW method takes advantage of the fact that the read element and write element of the magnetic head are at different positions along the radius (called the read-write offset in these specifications), to position the magnetic head while the read element reads patterns previously written on the inner circumferential or outer circumferential side, and the write element writes new patterns on the desired track separated by the read-write offset. Besides the product servo pattern, the SSW also writes other types of patterns on the recording surface, and utilizes these patterns to control the head positioning and the timing.
The HDD usually contains multiple recording surfaces, and multiple magnetic heads matching these recording surfaces, and an actuator supporting the multiple magnetic heads. The SSW method selects one magnetic head (called a propagation head in these specifications) from among the multiple magnetic heads to read the pattern on the recording surface, and positions the multiple magnetic heads by utilizing the signal on this read out pattern to control the actuator. All the magnetic heads simultaneously write the product servo patterns on the recording surfaces while positioned in this way. The SSW method is described in the following documents: U.S. Pat. No. 5,485,322; U.S. Pat. No. 5,581,420; U.S. Pat. No. 5,612,833; U.S. Pat. No. 5,615,058; U.S. Pat. No. 5,659,436, Japanese Patent Publication No. 1992-103023, and “A self-servowrite clocking process,” IEEE Trans Magn., 37, No. 4, pp. 1878-1880 (2001). A description within one or more of these references includes the propagation pattern and timing pattern.
Progress in giving the HDD a higher recording density on the other hand is usually achieved by enhancing the magnetic disk performance and making the track width of the magnetic head narrower. The ultra-thin film making up the magnetic layers as well as the tinier magnetic particles on the magnetic disk cause problems such as magnetization erasure or namely, thermal decay to appear that PMR (perpendicular magnetic recording) attempts to reduce. Some magnetic heads on the other hand have the problem of side-writing where unwanted recording occurs on adjacent tracks due to the spreading magnetic field from the write pole side surface during writing. Moreover, during read, the problem occurs that signals from adjacent tracks are read causing the problem of cross talk noise. These side-write and cross talk problems are difficult to resolve in currently used magnetic recording methods.
Discrete track recording (DTR) is a patterned media technology for alleviating the above problems. The discrete track media (DTM) for this type of recording utilizes the latest nanofabrication technologies such electron beam lithography and imprinting to form patterning on physically and magnetically isolated recording tracks. This discrete track recording suppresses problems such as side-write and cross talk and improves the signal quality.
Besides the discrete track media, bit patterned media (BPM) is also available. BPM also utilizes nanofabrication technology to form patterns the same as DTM. However, the main difference versus DTM is that BPM, allows patterning of recording bits (one bit of magnetic recording is equivalent to a one particle structure) that are physically and magnetically isolated from each other.
Fabricating DTM or BPM or in other words forming servo patterned media (patterned disk) requires pre-forming the servo pattern (servo track) and the track pattern (data track) simultaneously. Servo patterns formed in advance in this way are called pre-patterned servos. If the servo track and the data track are formed separately, then the track centers cannot be aligned so any positioning on a data track by the servo patterns will be meaningless. In other words, accessing the user data is impossible.
Besides the above described pre-patterned servo, another method for making a patterned disk is next described. Namely, a data track holding user data is fabricated as discrete tracks or as bit patterns, and a flat section left remaining without forming any patterns in order to magnetically write on the servo area. The servo can later write on this servo region (hereinafter “plane region”). This technology is described in “A self-servowrite clocking process,” IEEE Trans Magn., 37, No. 4, pp. 1878-1880 (2001).
However, various problems clearly emerge in processes where attempting to form the servo track or namely the servo pattern by utilizing electron beam lithography. The SSW method clearly provides the best features when servo characteristics of a disk containing a pre-patterned servo made in advance are compared with a disk magnetically written with servo tracks (servo patterns) by SSW.
Results from investigating causes of deterioration in pre-patterned servo characteristics clearly revealed the following three causes. A first cause is that the read waveform amplitude from the pre-patterned servo is small, and only approximately one-half the amplitude of the servo track magnetically recorded (written) by SSW so the playback (or read) signal-to-noise (S/N) ratio is extremely small. In other words, servo patterns can be magnetically recorded by utilizing the magnetic N pole and S pole but this does not work the same on the pre-patterned servo. In the pre-patterned servo, the servo patterns are formed by physically patterning magnetic units (magnetic thin film) on the disk so that only the magnetic N pole and nothing (or the S pole and nothing) are utilized on the disk.
A second cause is that forming an intricate servo pattern by electron beam lithography is difficult. In other words, the intricate patterning that makes up each servo pattern component such as the PLL, cylinder ID, sector No. and burst pattern within the servo pattern, or namely the pattern dimensions, shape, position accuracy, and other items are worse than those recorded magnetically.
A third cause is an innate problem or namely to what extent the combination of magnetic head and servo pattern match each other. The SSW method is implemented within the HDD so the magnetic head used for forming the servo pattern (servo track write process) is absolutely the same as the head of the actual HDD. The servo track pitch and the dimensions of each servo pattern are in other words exactly the ideal values for the SSW magnetic head unit. The pre-patterned servo on the other hand is determined beforehand by the disk dimensions so one can see that only a magnetic head within a specified range of numerical values can be used.
The above reasons show that the SSW method is superior to the pre-patterned servo method. The SSW method was next used to attempt writing servo patterns on plane regions formed in advance on the disk.
However, attempting to record (write) servo patterns on plane regions preformed on the disk revealed that mainly two large problems occur. One problem lies with along the disk circumference. The servo pattern and the track pattern must be formed so that they are consecutively and accurately positioned in a direction towards the disk periphery (or circumference). Though not occurring in magnetic servos and pre-patterned servos, a method that magnetically records servo patterns later on in the plane region has the problem that at what timing to start magnetic head excitation (or in other words, start servo writing) is unknown unless some type of disk circumferential position information is available. The magnetic head must therefore be supplied with timing information relating to the disk circumference.
Another problem lies along disk radius. The magnetic head must servo-write a servo track (or servo pattern) along the locus of a data track made up of a bit pattern or a discrete track made by pre-patterning. However when a disk pre-patterned with data tracks is attached to a spindle motor, the data tracks will possess an eccentricity as large as several dozen micrometers. In other words, the center of the spindle motor rotation will deviate from the center of the data track locus, so that the magnetic head is not slaved to the data track, consequently causing the problem that servo writing along the data track is impossible. This eccentricity is also a possible reason why achieving a perfectly round (nanometer scale) data track pattern is impossible. A servo write method is therefore needed that is capable of countering the eccentricity occurring along the disk radius.